1. The Field of the Invention
The present invention relates to a method and apparatus for the analysis of chemical compounds, and more particularly, to a novel method and apparatus for pyrolyzing compounds preliminary to analysis in a mass spectrometer.
2. The Prior Art
Many scientific techniques have been developed for the identification of chemical compounds; they include, for example, mass spectrometry, gas-liquid chromatography, infrared spectrometry, and nuclear magnetic resonance spectrometry. It is difficult, however, to use such techniques for the identification or "fingerprinting" of complex macromolecular materials.
One method advanced for fingerprinting such complex compounds is pyrolysis gas-liquid chromatography ("Py-GLC"). In the Py-GLC method, the sample to be analyzed is first "pyrolyzed" (i.e., chemically decomposed by heat) before being introduced into the gas-liquid chromatograph. The products resulting from pyrolysis are usually of lower molecular weight than the original macromolecular structure of the sample, and are, therefore, more easily analyzed by the gas-liquid chromatographer.
Although significant progress has been made in the development of Py-GLC techniques during recent years, the usefulness of these techniques for the fingerprinting of organic solids and other complex materials is still considerably limited by a lack of standardization and interlaboratory reproducibility. The reproducibility problems encountered are due primarily to the practical imperfections of gas chromatography rather than to problems associated with the pyrolysis techniques. For example, such factors as insufficient column resolution, gradual deterioration of column performance, or sudden differences introduced by column replacement have heretofore posed formidable obstacles to the computer processing of Py-GLC data and the compilation of reference libraries of standard fingerprints.
The need for reproducible fingerprints of complex biological and geochemical materials has thus led some to explore the technique of pyrolysis mass spectrometry. Using this technique, a sample having a complex macromolecular structure is first pyrolyzed to break the structure down into lower molecular weight, volatile fragments characteristic of the original structure. These fragments are then ionized and analyzed by mass spectrometric techniques.
The repeatability of pyrolysis fragmentation patterns is, among other considerations, determined by the reproducibility of the temperature rise profile while heating the sample, and the maximum temperature at which pyrolysis occurs. These parameters can be standardized by using ferromagnetic wires to hold the sample and energizing a high frequency coil to heat the wire to its Curie-point. At the Curie-point of the ferromagnetic wire, the disappearance of the ferromagnetic properties of the wire stabilizes the maximum wire temperature. Such a Curie point technique thus begins to solve some of the problems associated in obtaining the reproducibility needed for identification purposes.
Although recent developments in pyrolysis mass spectrometry have eliminated some of the problems associated with pyrolysis gas-liquid chromatography, significant problems are still encountered during the pyrolysis phase of the pyrolysis mass spectrometric technique. For example, due to the high reaction chamber wall temperatures needed to prevent condensation losses of pyrolysis products, premature thermal damage to the sample may occur before pyrolysis can be performed. As a result, a different mixture of pyrolysis products is obtained which is less characteristic of the original sample. Furthermore, the ill-defined nature of the thermal damage to the sample aggravates reproducibility problems.
In order to avoid the problem of premature heating, most Curie-point pyrolysis studies use relatively cold reaction chamber walls. However, this creates another problem, namely the loss of less volatile pyrolysis products. Immediately after pyrolysis of the high molecular weight sample, the less volatile components of the pyrolyzed sample condense on the relatively cold walls of the reaction chamber. Thus, these less volatile components remain within the reaction chamber and are not subsequently analyzed by the mass spectrometer. Not only does this problem render the resultant mass spectrum incomplete, but it also contaminates the reaction chamber so that subsequent sample analyses may be inaccurate.
Moreover, both the effects of premature sample heating and of condensation losses on the reaction chamber walls may change from sample to sample. Hence, the reproduibility of the spectra obtained from the mass spectrometer is greatly impaired, and standardization of the spectra for subsequent identification referencing becomes difficult. It will be readily appreciated that the loss or change of components in the mass spectra creates missing pieces in the puzzle for determining the overall structural composition of the material analyzed. If this problem were eliminated, mass spectrometric analysis would provide a better description of the entire macromolecular structure, thus aiding the scientist in deducing both the chemical elements present and the structural composition of the analyzed sample.
A further problem associated with the various apparatus implementing the pyrolysis phase of pyrolysis mass spectrometry is that generally, each apparatus employs one fixed mode of limited application. For example, one typical pyrolysis/mass spectrometer employs only the rapid heating mode described above. In this type of apparatus, the sample is placed on a ferromagnetic wire which is heated by induction to pyrolyze the sample. In another apparatus, the sample is introduced into a reaction chamber and heated gradually until chemical decomposition of the sample is achieved. This mode is often referred to as "direct probe" or "oven pyrolysis." Still another apparatus pyrolyzes the sample by means of a laser beam.
Additionally, pyrolysis mass spectrometers employ different techniques for introducing the decomposed components of the sample into the mass spectrometer for analysis. In one apparatus, an expansion chamber is positioned between the pyrolysis reaction chamber and the mass spectrometer to delay passage of the pyrolyzed products into the mass spectrometer and thus obtain one mass spectrum of the entire pyrolyzed sample. Another pyrolysis mass spectrometer has no such expansion chamber--pyrolysis of the sample being achieved directly in front of the ion source of the mass spectrometer. This type of pyrolysis apparatus allows the scientist to obtain mass spectra of the individual components of the pyrolyzed sample produced at different times during pyrolysis, since the volatile pyrolysis products are not mixed and delayed in an expansion chamber. However, highly volatile components produced in this type of pyrolysis apparatus often escape analysis by the mass spectrometer due to the minimum response time required by the mass spectrometer for analysis thereof.
Thus, the pyrolysis mass spectrometers existing in the art are generally each capable of implementing only one mode of pyrolysis--the heating and pyrolysis product introducing features being fixed and invariable.
In view of the foregoing, it would be a significant advancement in the art to provide a pyrolysis inlet for a mass spectrometer that would protect the sample from thermal damage prior to pyrolysis while pyrolyzing high molecular compounds without significant condensation of low volatile pyrolysis products on the reaction chamber walls. Additionally, it would be another desirable advancement in the art to provide a pyrolysis inlet for a mass spectrometer which is capable of implementing several pyrolysis modes, as well as sample introduction modes, into a single apparatus. Such an invention is disclosed and claimed herein.